Processing materials inside an atmospheric-pressure radiofrequency nonthermal plasma discharge

ABSTRACT

Apparatus for the processing of materials involving placing a material either placed between an radio-frequency electrode and a ground electrode, or which is itself one of the electrodes. This is done in atmospheric pressure conditions. The apparatus effectively etches or cleans substrates, such as silicon wafers, or provides cleaning of spools and drums, and uses a gas containing an inert gas and a chemically reactive gas.

The present invention generally relates to material processing and, morespecifically, to the processing of an object or material by insertionbetween the electrodes of an atmospheric-pressure radiofrequencynonthermal plasma discharge. This invention was made with Governmentsupport under Contract No. W-7405-ENG-36 awarded by the U.S. Departmentof Energy. The Government has certain rights in the invention.

Surface cleanliness is of vital importance in many industries, not theleast of which is the semiconductor industry. Clean substrates anddevices are imperative if high quality devices are to be manufactured.Additionally, other applications require that materials be etched in apredetermined manner to effect the desired operation from the material.

Currently, this cleaning or etching is accomplished through a variety ofmethods. Among these methods are low-pressure plasma processing andatmospheric-pressure RF plasma discharge processing. By utilizing thereaction of the plasma with selected feed gases, surface processing hasbeen performed on organic films, fabrics, and semiconductor wafers. Inthese processes, the target material is immersed in the plasma,typically by placing the workpiece directly on an electrode.

These discharge processes are effective because of the action of theions, which typically are more chemically active than the correspondingneutral gas species, due to their greater collision cross section andreaction probability. Also, ions are accelerated across the sheathregion in the plasma. This gives rise to the directed flux of positiveions onto the workpiece. The kinetic energy contributed by these ionscombined with their chemical reactivity results in the desired chemicalreaction. Unfortunately, however, these prior art processing methodsrequire expensive vacuum systems in order to be effective, because theformation of a sheath is favored by low pressure and to obtain a highkinetic energy of the ions it is necessary to minimize gas-phasecollisions within the sheath region.

The primary objective of the present invention, like the priordiscipline of low-pressure plasma processing, is to modify selectedsurfaces. This modification can include contamination removal, surfacematerial removal, known as etching, or changes in the physical state orproperty of the surface, known as surface modification.

Previous demonstrations of atmospheric pressure, RF plasma dischargethat are related to the present invention are U.S. Pat. No. 5,961,772,issued to Gary S. Selwyn for “Atmospheric-Pressure Plasma Jet,” and U.S.patent application Ser. No. 09/295,942, filed Apr. 21, 1999, for “LargeArea Atmospheric-Pressure Plasma Jet.” The first involves a coaxial,cylindrical electrode configuration, and the second a parallel plateelectrode configuration. In both of these demonstrations, the plasmaby-products are blown out of the source region, in which the plasma isgenerated, and is directed against a surface to be treated. The targetsurface is typically a few millimeters from the source.

It should be noted that radicals, and particularly ions, in the plasmadischarge are extremely short lived, and cannot be transported for longdistances outside the discharge region. Metastable species producedinside the plasma, on the other hand, have longer lifetimes atatmospheric pressure, typically on the order of hundreds ofmilliseconds. This longer lifetime allows them to be carried out of theplasma volume along with the gas flow and impinge against an externalmaterial or surface.

The fast flow of the reactive gas stream exiting the plasma volumeincreases the reaction distance, that is, the distance at which theplasma jet may be positioned from a workpiece and still provideeffective reaction chemistry. High gas flow also increases the flux ofreactive species onto the workpiece. To accomplish all of this, high gasflows must be maintained to carry reactive metastable and other plasmaspecies to the workpiece before they decay and become nonreactive. Ofcourse, high gas flow rates increase the cost of plasma processing byincreasing the cost of consumables, or require reprocessing of the spentgas.

Such downstream treatment of materials as described also has somedistinct advantages. It reduces the likelihood of surface damage to theworkpiece, because after exiting the plasma volume, most of the chargedspecies have recombined and are neutralized. One common source of damagein microelectronic devices is due to the build up of charge ondielectric on semiconductor surfaces. Therefore, surface charging is notan issue in downstream processing. Because neutral species are notaccelerated to high kinetic energy in the way that ions are in lowpressure plasma processing equipment, the reaction chemistry of neutralspecies is more selective, albeit slower, than ion chemistry.

Clearly, in cases where selectivity is not an important issue, such asthe removal of organic contaminants from silicon or metals (because ofthe innate huge difference in chemical reactivity between the former andthe latter two materials), the process rate could be improved byincorporating ion-driven chemistry into the reaction scheme. This may beaccomplished through direct immersion of the workpiece into thevolumetric plasma.

Because the plasma volume contains significant ionic components,cleaning and surface treatment of materials may be accelerated byutilizing ion-driven chemistry. Also, by immersing the workpiece intothe plasma, high gas flow rates are not needed to drive the reactivespecies several millimeters before they decay or recombine. This is dueto the fact that the reactive species are present immediately adjacentto the workpiece because they are formed in the same volume as theworkpiece.

This is the thrust of the present invention: to provide means for theutilizing ion chemistry even for a small plasma volume as is present inthe plasma jet source described above, but without the added risk ofsurface damage caused by high energy impact of the ions onto theworkpiece. In the present invention the workpiece is introduced directlyinto the plasma volume and is exposed to the ion and neutral chemistryof the plasma, and to the high-pressure environment of the plasma.

A clear advantage of this approach relative to the prior art oflow-pressure plasma processing equipment is that the high-pressureenvironment of the plasma limits the strength and dimension of thesheath, which in turn limits the kinetic energy of the ions. Ions areaccelerated less by the weaker, thinner sheath, and those ions thatimpinge the surface have lower kinetic energy as a result of the smallerelectric field in this sheath, as well as the frequent, gas-phasecollisions the ions undergo with neutral species. The resultant lowerkinetic energy of ions leads to less surface damage.

An advantage of the direct immersion process taught in this inventionrelative to the prior art of downstream, atmospheric pressure plasmaprocessing is that the reaction chemistry benefits from the addedpresence of ions, which would be recombined and therefore lost to thedownstream chemistry processing approach. Also, because gas flow is notneeded to carry the reactive species several mm to the workpiece, lowergas consumption is possible.

The workpiece may either be as rigid as a silicon wafer or as flexibleas manmade or natural textiles. Because the workpiece is exposed toradio frequency power in this invention, there is no limitation that itbe conducting at dc power: both dielectric materials and semiconductors,as well as conductors may be processed.

It is therefore an object of the present invention to provide apparatusand method for cleaning and processing materials inside a high-pressureplasma discharge.

It is another object of the present invention to provide apparatus andmethod for cleaning and processing materials that use less process gas.

It is yet another object of the present invention to provide apparatusand method for treating materials while creating less surface damage tothe materials.

It is still another object of the present invention to provide a meansof treating materials that may be conductors, semiconductors ordielectric in nature.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, the apparatus of this invention for the plasma processing ofmaterials in an atmospheric pressure radio-frequency non-thermal plasmacomprises an electrically conductive enclosure defining an interiorspace with a surface and with openings for introduction of a gas and forentry and exit of a material to be processed, with an electrode situatedinside the interior space and spaced apart from the surface of theinterior space a distance sufficient to allow placement of the materialto be processed. Means for placing the material to be processed islocated inside the interior space between the electrode and theelectrically conductive enclosure. When a gas is introduced into theinterior space through the opening for introduction of a gas and aradio-frequency voltage is applied between the electrically conductiveenclosure and the electrode, a plasma is created in the interior spacefor processing the material to be processed within the electricallyconductive enclosure.

In another aspect of the present invention and in accordance with itsprinciples and purposes apparatus for processing materials in anatmospheric pressure radio-frequency non-thermal plasma comprise anelectrically conductive enclosure defining an interior space with asurface and inlets for a gas and for entry and exit of a material to beprocessed with an electrode spaced apart from the electricallyconductive enclosure and capable of placing the material to be processedinside the interior space between the electrically conductive enclosureand the electrode, the material to be processed being in contact withthe electrode. When a gas is introduced into the inlet for gas and aradio-frequency voltage is applied between the electrically conductiveenclosure and the electrode a plasma is created in the interior spacefor processing the material to be processed as it passes through theelectrically conductive enclosure In a still further aspect of thepresent invention and in accordance with its principles and purposesapparatus for processing materials in an atmospheric pressureradio-frequency non-thermal plasma comprise a grounded enclosuredefining a first interior space, gas inlet and outlets and an openingfor radio-frequency voltage connection, with a radio-frequency connectorin said opening. A radio-frequency electrode is located in the interiorspace in electrical contact with the radio-frequency connector, anddefines an opening for the gas inlet and a second interior space.Grounded means for retaining a spool of material to be processed is inclose proximity to the radio-frequency electrode. When a gas isintroduced through the gas inlet and a radio-frequency voltage isapplied between the radio-frequency connector and ground, plasma iscreated between the radio-frequency electrode and the spool of materialto be processed thereby providing cleaning of the spool of material.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is a schematic illustration of one embodiment of the presentinvention in which the material to be processed is pulled by rollerthrough the volume between a ground electrode and a radio frequencyelectrode.

FIG. 2 is a schematic illustration of another embodiment of the presentinvention in which the radio frequency electrode is used as a roller topull the material to be processed into the volume between the radiofrequency electrode and the ground electrode.

FIGS. 3A and 3B are plots of voltage versus plasma current for severalgas flow rates.

FIG. 4 is an illustration of an embodiment of the present inventionemploying cylindrical coaxial electrodes.

DETAILED DESCRIPTION

The present invention provides atmospheric pressure plasma processing ofmaterials in an effective and efficient manner. The invention can beunderstood most easily through reference to the drawings.

In FIG. 1 there is a cross-sectional schematic illustration of oneembodiment of the invention where material plasma processor 10 defineselectrically conductive enclosure 11 that forms an enclosed volume 11 ahaving gas inlet 11 b, material inlet 11 c and material outlet 11 d.Inside volume 11 a, roller 12 and electrode 13 are situated. Roller 12serves to pull film or textile material 14 through enclosed volume 11 abetween electrically conductive electrode 11 and electrode 13. Thosewith skill in this art will appreciate that any appropriate means otherthan roller 12 could be used to transport film material 14 into placeinside enclosed volume 11 a. Roller 12 could also be the RF poweredelectrode in some cases.

With an appropriate gas injected through gas inlet 11 b and theappropriate level of RF voltage applied either to electricallyconductive enclosure or to electrode 13 with respect to the otherserving as a ground electrode, a plasma will be created in enclosedvolume 11 a for processing said film material 14 as it is pulled throughground electrode 11 by roller 12. The appropriate gas used can be anygas that can provide the proper ion-driven chemistry for the intendedprocessing. In normal operations, an inert gas is the major gascomponent, along with the addition of a reactive gas such as oxygen inan appropriate amount. However, other gases may also be added, subjectto the arcing performance of the plasma source. In the preferredembodiment, a gas mixture consisting of 99% helium+1% oxygen atatmospheric pressure is used to remove organic contaminants from metalor silicon surfaces.

The outlet for the gas introduced into enclosed volume 11 may simply besmall openings between the components that comprise the electricallyconductive enclosure, or they may be tubing used for gas reprocessing orexhaust. This is true for all of the embodiments of the presentinvention described herein.

It is important to note that inasmuch as the present invention utilizesRF energy to create a plasma and to process materials, electricallyconductive enclosure 11 does not necessarily need to be grounded. Insome circumstances it may be desirable to have electrically conductiveenclosure 11 floating and apply RF energy 15 at some predeterminedphase, which can differ by as much as 180°, with respect to RF energy 16applied to electrode 13, to enhance the effectiveness of the processing.In this situation, a protective, grounded casing 14, shown by dashedlines in FIG. 1, would enclose the invention for safety reasons. Anappropriate frequency for the RF energy used in the present invention is13.56 Megahertz (MHz), however other RF frequencies might also proveuseful.

FIG. 2 illustrates another embodiment of the present invention wherematerial plasma processor 20 defines electrically conductive enclosure21 that forms enclosed volume 21 a, gas inlet 21 b, and material inlet21 c and material outlet 21 d. Inside enclosed volume 21 a, electrode 22also serves as a roller, such as roller 12 of FIG. 1. However, as withthe previous embodiment, it is not necessary that electrode 22 be in theform of a roller. Any other appropriate configuration can be used aslong as it is capable of placing a material inside enclosed volume 21 a,between electrically conductive enclosure 21 and electrode 22, and incontact with electrode 22

In this embodiment, any material inserted into enclosed volume 21 athrough material inlet 21 c and is placed in or is pulled throughenclosed volume 21 a by, or on top of electrode 22. In this embodiment,electrode 22 is in direct contact with the material to be processed,making the material the part of the electrode. In this embodiment, thematerial can receive the full effect of all of the plasma products. Itshould be noted that even dielectric or semi-conducting substrates couldbecome part of electrode 22 and subject to ion impingement, as RFfrequency will penetrate such media.

The present invention provides direct immersion of the material into theplasma, providing an important advantage over the plasma processingtechniques of the prior art. As an example, the short-lived speciespresent in the plasma volume, such as ions and certain radicals canattack the material's surface because they are present within thediffusion distance of the material's surface. Additionally, since thegas does not need to flow at high velocity in order to carry reactivespecies beyond the exit of the plasma source, the gas flow rate can bereduced significantly. This results in savings in the cost of theprocess gas and the overall processing cost.

The overall low gas flow rate of the present invention, a few standardliters per minute (slpm), addresses a limitation of the prior artAtmospheric Pressure Plasma Jet, namely large usage of He gas formaintaining an arc-free discharge. The present invention is based on astudy of the discharge electrical properties of the plasma jet as afunction of total gas flow rate. As shown in FIGS. 3A and 3B, the stableregion of plasma discharge does not change appreciably as the total gasflow rate is decreased from 40 slpm to 2.5 slpm, a factor of 16, withthe gas composition remaining constant. These data indicate theexistence of a stable region of plasma discharge even at very low gasflow rates, so long as the gas composition remains constant. The gascomposition easily can be maintained in an airtight (not a vacuum)environment.

This low gas flow airtight environment is the hallmark of the previouslydescribed embodiments of the present invention. Low gas flow ratedecreases the cost of the process and makes treatment of relativelylow-value added processes, such as textile treatment, economicallyviable. By insertion of the material to be processed into the plasmadischarge zone as is done in the embodiments of the present invention,the maximum benefit of the plasma is achieved. Compared to the prior artAtmospheric Pressure Plasma Jet, which relies on metastable and otherlong-lived neutral species for chemical reactions outside the jet, thepresent invention provides in-situ material processing utilizing thefull potential of the atmospheric pressure plasma discharge, includingcharged species or ions, atomic and radical species, as well aspotentially the UV radiation emitted by the plasma to aid materialprocessing. Of course, the contribution of the metastable and otherlong-lived species of the effluent-based plasma treatment of the priorart also is of value to the processing by the present invention, asthese are still present within the discharge region.

Referring again to FIG. 2, it should be understood that this embodimentof the present invention, in having the material to be processed incontact with electrode 22, maximizes chemical reactivity of the plasmawhile at the same time allowing temperature control of the material.This can be accomplished by simply heating or cooling the electrode incontact with the material. This temperature control ability can be usedto enhance the rate of chemical reaction or to limit any detrimentalside effects such as thermal damage to the material to be processed.

To verify the efficacy of this embodiment, a KAPTON® film was processedthrough enclosed volume 21 a using He at a flow rate of 42 slpm and anO₂ flow rate of 0.36 slpm, and an input power of 345 watts. KAPTON® is aflexible, dielectric film comprised of polyimide. The gap spacingbetween ground and a flat 10 cm by 10 cm stainless steel RF electrodewas 0.16 cm. With this configuration, an etch rate of up to 9 mg perminute was measured both for the KAPTON® film between the electrodes andfor the KAPTON® film in contact with the electrodes.

Another embodiment of the invention is illustrated in a cross-sectionalview in FIG. 4, in which outer enclosure 41 defines enclosed volume 42in which conformal electrode 43 encloses electrically conductive object44. Electrically conductive object 44 represents any spool or objectthat is in need of cleaning, such as a printing press roll, or acylinder intended for recycling from a laser-printing cartridge or aspool used for treatment of thread material in need of cleaning.Electrically conductive object 44 is retained inside conformal electrode43 by physical connector clamp 45 and threaded shaft 46. In thisembodiment, electrically conductive object 44 is grounded and functionsas the grounded electrode. In this case, it is preferable to groundelectrically conductive object 44 and to position an axially symmetric,RF-powered electrode concentric to electrically conductive object 44 inorder to form a plasma. Because electrically conductive object 44 can begrounded, it may be left attached to other equipment without damagingthe connected equipment by passage of the RF current. However, in othercircumstances, electrically conductive object 44 may be RF powered andconformal electrode 43 may be grounded or may be RF powered at adifferent phase than is electrically conductive object 44

Outer enclosure 41 provides an opening for gas tube 47 and for RFconnector 48 that provides electrical connection to conformal electrode43. Outer enclosure 41 also provides viewing ports 49, as does conformalelectrode 43. Thermocouple clamp 50 retains a thermocouple forcontrolling heater 51 for maintaining an appropriate temperature ofelectrically conductive object 44.

The foregoing description of the embodiments of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously many modifications and variations are possiblein light of the above teaching. The embodiments were chosen anddescribed in order to best explain the principles of the invention andits practical application to thereby enable others skilled in the art tobest utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto.

1-25. (canceled)
 26. Apparatus for processing materials in anatmospheric pressure radio-frequency non-thermal plasma consisting of:an electrically conductive enclosure defining and interior space with asurface and openings for introductions of a gas and for entry and exitof a material to be processed while said interior space is at or nearatmospheric pressure; an electrode situated inside said interior spaceand spaced apart from said surface of said interior space a distancesufficient to allow placement of said material to be processed; amechanical action for placing said material to be processed inside saidinterior space on said electrode or between said electrode and saidelectrically conductive enclosure; and, a radio frequency power supplyhaving a phase applied between said electrode and said electricallyconductive enclosure; wherein a gas containing a majority of inert gasis introduced into said interior space through said opening forintroduction of a gas creating an atmospheric pressure plasma in saidinterior space for processing said material to be processed within saidelectrically conductive enclosure.
 27. The apparatus as described inclaim 26, wherein said phase has a frequency of 13.56 Megahertz.
 28. Theapparatus as described in claim 26, wherein said electrode and saidelectrically conductive enclosure are cylindrically shaped.
 29. Theapparatus as described in claim 26, wherein said electrode is a rotatingroller.